There are two types of imaging apparatuses that have a solid-state image sensor such as a CCD (Charge Coupled Device) image sensor. The first type has one CCD image sensor (hereinafter, referred to as “single-plate camera”). The second type has three CCD image sensors (hereinafter, referred to as “three-plate camera”).
In a three-plate camera, the three CCD image sensors for generating an R signal, a G signal and a B signal, for example, generate three primary-color signals, respectively. The three primary-color signals are processed, whereby a color image signal is generated. The color image signal is recorded in a recording medium.
In a single-plate camera, the CCD image sensor is arranged in front of a color-coding filter that comprises a color-filter array composed of color filters, each allocated to one pixel. The color-coding filter generates color-coded, color component signals, each for one pixel. The color-filter array, which constitutes the color-coding filter, includes primary-color filters such as a R (Red) array, a G (Green) array and a B (Blue) array and complementary-color filters such as a Ye (Yellow) array, a Cy (Cyanogen) array and an Mg (Magenta) array. In the single-plate camera, the CCD image sensor generates a signal representing one color for one pixel, and another signal representing another color is generated for the pixel by means of linear interpolation, thereby providing an image that is similar to the image the three-plate camera provides. The single-plate camera is incorporated in a video camera or the like, which should be small and light.
The CCD image sensor provided in a single-plate camera may be arranged in front of a color-coding filter that comprises a color-filter array of such color arrangement as is shown in FIG. 1A. Each pixel of CCD image sensor, arranged at the back of a color filter, outputs only an image signal that corresponds to the primary color R, G or B, of that color filter. That is, a pixel positioned at the back of an R filter outputs an R-component image signal, but cannot output a G-component image signal or a B-component image signal. Likewise, a G-component pixel outputs only a G-component image signal, but cannot output an R-component image signal or a B-component image signal. A B-component pixel outputs only a B-component image signal, but cannot output an R-component image signal or a G-component image signal.
The color arrangement of the color-filter array, which is shown in FIG. 1A, is called “Bayer arrangement.” In this case, G-color filters are arranged in the pattern of a chessboard, and R-color filters and B-color filters are arranged in alternate columns, each in a vacant square.
An R-component signal, a G-component signal and a B-component signal must be processed for each pixel in the next-stage section. To process these signals, interpolation has hitherto been performed on the outputs from a CCD image sensor having n×m pixels (n and m are positive integers). Thus, n×m R-pixel image signals and n×m G-pixel image signals are generated as is illustrated in FIG. 1B. That is, image signals equivalent to those CCD outputs of a three-plate camera are output to the next-stage section.
To generate image signals in, for example, density four times as high, interpolation is performed, generating 2n×2m R-pixel image signals from the n×m R-pixel image signals, 2n×2m G-pixel image signals from the n×m G-pixel image signals, and 2n×2m B-pixel image signals from the n×m B-pixel image signals.
In the above-described single-lens camera, however, a linear process is carried out to interpolate color signals. The waveform of the image is inevitably deformed, rendering the image unclear as a whole. A process such as edge emphasis must therefore be performed to increase the apparent resolution of the image. Since the image resolution achieved by the image signals output from the single-plate camera is lower than the image resolution attained by the outputs of a three-plate camera, the resultant image is blurred as a whole due to the influence of the linear process.
The three primary-color components of each pixel, which have the same resolution, may be generated from an output of the CCD image sensor of the single-plate camera, thus obtaining image signals. Image signals may then be obtained in a higher density from the image signals thus generated, thereby to increase the pixel density. This method, however, cannot provide a sufficient precision.
It is proposed that the classification-adaptation process, i.e., a process other than linear interpolation, be performed on the CCD outputs of the single-lens camera, for each of the R, G and B primary-color image signals, thereby to generate image signals that are equivalent to the CCD outputs of a three-plate camera. (See Japanese Patent Application No. 8-508623.) However, when the classification-adaptation process is effected on the R, G and B primary-color image signals independently, the same process is performed on each R pixel and each B pixel as on each G pixel (two existing in very four pixels), though only one R pixel exists in very four pixels and only one B pixel exists in very four pixels in the m×n pixels as shown in FIGS. 1A and 1B in the case where a color-filter array of Bayer arrangement is used. Consequently, high-precision prediction cannot be accomplished as far as the R-component signals and B-component image signals are concerned.